Field of the Invention
The present invention relates to spread spectrum communication apparatus and method for receiving a wide-band spread spectrum signal.
FIG. 2 shows a spread spectrum communication apparatus.
A received spread signal is supplied from one of antennas 111 and 112 which is selected by an antenna switch 110, to a low-noise amplifier 113 via a band-pass filter (BPF) 109. Then, a frequency of the received signal is converted into a predetermined frequency by a frequency converter 114 to which a local oscillator 102 is connected. The received signal which has been subjected to the frequency conversion is further subjected to predetermined band restriction by a band-pass filter (BPF) 115, and is then supplied to a variable amplifier 116. After then, the received signal of which level has been set at a predetermined reception level is supplied to a demodulator 117 to be demodulated to data S3, and the data S3 is subsequently supplied to a control unit 118.
In this case, an output from the variable amplifier 116 is also supplied to a wave detector 119 to detect field intensity of the received signal. A voltage detected by the wave detector 119 is applied to an automatic gain control (AGC) voltage generator 120, such that the applied voltage acts as a control voltage of the variable amplifier 116 to set the level of the received signal as a predetermined signal level. Also, an output from the AGC voltage generator 120 is supplied to an antenna switch signal generator 121 to switch or change the antenna in accordance with a signal S7 in a case where a level of such output is equal to or smaller than a predetermined threshold level.
However, in the spread spectrum communication apparatus shown in FIG. 2, since wave detection is performed by the wave detector 119 for an entire band (i.e., a band of the BPF 115) of the received signal, if there are noise components, e.g., an unnecessary wave included in the band, a cross modulation wave generated in a system, and the like, voltage levels of these noise components are also detected, whereby it is difficult to detect a normal signal level. Especially, as the level of the received signal becomes lower, an amplification factor of the variable amplifier 116 becomes larger, whereby only noise power is further amplified. Therefore, the level of the received signal can not be accurately detected. As a result, it is difficult to switch the antenna normally.
Further, since the received signal is a wide-band spread signal, a fall of spectrum within the band has various forms because of multipath. In this case, the multipath represents a phenomenon in which there are a plurality of paths, e.g., a reception side receives a direct wave and a reflected wave or receives a plurality of reflected waves.
FIGS. 3A to 3C respectively show states of the fall of spectrum of the received signal because of the multipath.
FIG. 3A shows the state that there is no fall of spectrum of the received signal because of the multipath, FIG. 3B shows the state that there is a fall of spectrum at a frequency adjacent to a central frequency f0 (i.e., apart from the central frequency f0 by f1), and FIG. 3C shows the state that there is the fall of spectrum at the central frequency f0. In the case of FIGS. 3B and 3C, in the apparatus shown in FIG. 2, if received power is uniform within the band, the wave detector 119 detects the same-level voltage.
However, error generation probability in the case of the fall of spectrum shown in FIG. 3B tends to be higher than that in case of the fall of spectrum shown in FIG. 3C. That is, as the fall of spectrum adjacent to a main lobe of the received signal is larger, the error generation probability tends to be higher.
Therefore, in the apparatus shown in FIG. 2, since the received signal is subjected to the wave detection for the entire band, differences of the fall of spectrum cannot be judged. Thus, there is a drawback that the antenna is switched only based on the field intensity of the received signal.
FIG. 7 shows the structure of a conventional spread spectrum communication apparatus which relates to transmission power control.
In FIG. 7, a code generator 7 generates a pseudo-noise (PN) code for use in de-spreading, and a multiplier 3 multiplies a received signal by an output of a local oscillator 10. An automatic gain control (AGC) voltage generator 19 outputs a control signal to a variable gain amplifier on a reception side. Also, the AGC voltage generator 19 outputs the control signal to a variable gain amplifier 13 on a transmission side.
In this manner, if a correlation output is used for transmission power control, there is a drawback that a long time is required.
Such drawback also occurs in a case where the correlation output is used for switching the antenna.